"The Postal Service's financial crisis will likely come to a head in the next few months," they added. "Without legislation, the Postal Service will not be able to make payments that are due and will likely be forced to slash services."

The letter does not ask the House to approve the Senate-passed bill, S. 1789, but does indicate that the senators clearly prefer their bill as a way forward. The senators said that rather than risk USPS closures, their bill would give the service access to money it has overpaid into its retirement fund, and use that money to encourage the retirement of 100,000 workers, which would lead to long-term savings.

The Senate would also reduce the amount the USPS would have to fund its future retiree health benefits, and delay for two years any move to a non-Saturday delivery schedule.

The Senate bill, which passed last week in a 62-37 vote, is very different from the leading House GOP bill, H.R. 2309. That proposal, from House Oversight and Government Reform Committee Chairman Darrell Issa (R-Calif.), would set up a commission to recommend closures, authorize USPS to move to a five-day delivery system, and end a current rule that prohibits the closure of an office solely because it's operating at a deficit.

Among other things, the bill would terminate a requirement that postal worker benefits meet a standard set in 1971.

House Republicans are expected to press for Issa's bill, especially in the wake of criticism of the Senate bill, which many Republicans said would only delay the needed reforms USPS needs to take to get back into the black.

"As even the Postmaster General has said, the postal service needs more meaningful reform than the Senate bill provides," BoehnerJohn Andrew BoehnerDems face hard choice for State of the Union response Even some conservatives seem open to return to earmarks Overnight Finance: Trump, lawmakers take key step to immigration deal | Trump urges Congress to bring back earmarks | Tax law poised to create windfall for states | Trump to attend Davos | Dimon walks back bitcoin criticism MORE spokesman Brendan Buck told The Hill. "The House, led by Chairman Issa, is committed to passing legislation that offers the postal service a pathway to long-term solvency, not simply a bailout."

The May 15 deadline already has the makings of another showdown between House Republicans and a Democrat-led group in the Senate. Carper on Tuesday announced a clock that counts down the days before USPS starts closing facilities, and says the failure of the House to act by May 15 "could accelerate an already deteriorating financial situation" at the USPS.